Apparatus for timing x-ray exposures



July 25, 1939. F. N. SCAIFE 2,167,529

APPARATUS FOR TIMING X-RAY EXPOSURES I Filed Aug. 12, 1937 grmeA/fy Patented July 25, 1939 UNITED STATES PATENT OFFICE APPARATUS FOR TIMING X-RAY EXPOSURES Application August 12,

1937, Serial No. 158,643

In Great Britain October 9, 1936 3 Claims.

This invention relates to apparatus for timing X-ray exposures. These exposures, particularly for photographic purposes, are usually timed by means of a clockwork mechanism, the clockwork 5 being set in operation simultaneously with the closure of the tube circuits and serving to operate a switch to extinguish the discharge at the conclusion of the desired period.

Such mechanically operated time switches have several disadvantages. They are usually noisy in operation, and the release of the mechanism often causes the patient, who is usually in a highly nervous condition, to jump and thus spoil the film. The timing apparatus must be capable of being set for exposures of widely varying duration and is usually set to operate for times up to ten seconds. For short exposures a small movement only of the indicator is necessary and it is thus diificult to set it accurately.

To avoid these disadvantages we make use of the principle of regulating the exposures by means of the time necessary to charge a condenser up to the breakdown voltage of a gaseous discharge tube.

Such apparatus is silent in operation and is capable of being set accurately for both long and short exposures.

Preferably the charging current for the condenser is made dependent upon the emission of the cathode of the X-ray tube, thus ensuring that the correct exposure is given independently of variations in the intensity of the X-ray emission due to fluctuations in the supply voltage and the like.

The nature of the invention will be better understood from the following description of one embodiment thereof taken in conjunction with the accompanying drawing.

Referring to the drawing, the necessary voltage 40 between the anti-cathode and cathode of an X- ray tube X is supplied from the secondary of a transformer T1. This secondary is divided into two halves, one end of the left-hand half being connected to the anti-cathode of the tube and the other end being grounded, for example, through the transformer core. A portion of the right hand half of the secondary of transformer T1 supplies the filament of the tube X with current and the end of this half remote from the connections to the filamentary cathode is connected through resistances R1 and Re to ground. The primary of transformer T1 is connected to the A. C. mains through contacts pimps of a switch. This switch is operated by means of a push button P in such a way that contacts 122 and 113 first close, connecting the primary of T1 to the mains through a resistance R5 and afterwards contacts 171 and p2 close, short circuiting resistance R5. The push button P is operated against a spring SP and held in operated position by a latch L connected to the armature of a relay B.

The electron current between the cathode and anti-cathode of tube X flows through resistances R1 and Re and the voltage thereby developed across these resistances is applied to a condenser C2 through either resistance R2 or resistance R3. The condenser C2 is shunted by a neon tube N and a relay A in series.

The values of the resistances R1 and Rs are preferably chosen so that the potential across them under normal operating conditions is about 1 times the discharge voltage of the neon tube N, These resistances are shunted by a condenser C1 to smooth out surges. The resistance R2 is variable in steps, each step representing a charging time of one second before the neon tube flashes and the resistance R3 is variable in steps, each step representing a charging time of 0.1 second before the neon tube flashes. The two switches for selecting the fractions of resistances R2 and R3 respectively to be connected in the circuit of condenser C2 are gauged together so as to be operated together by movement of a pointer on the outside of the casing moving over a scale. Either of the two resistances may be placed in circuit by means of a switch S.

On the voltage across the terminals of condenser Cz rising to a predetermined value, the neon tube N will discharge through relay A, which .energises, and closes by means of its contacts a a circuit for relay B, which in turn energises and mechanically trips the latch holding push button P in operated position. Contact 791 p2 and 103 are opened, and contacts 01 closed the latter causing any residual charge of the condenser to flow ofi through resistance R1.

It will be seen that an excessive emission current in the X-ray tube will produce a correspondingly greater voltage across resistances R1 and Re and the exposure time will be correspondly less. On the other hand, reduced emission will mean increased exposure time so that radiographically the exposures tend to approach equality irrespective of substantial variations in emission current.

What is claimed is:

1. Apparatus for timing X-ray exposures comprising an X-ray tube having a discharge path, connections for supplying operating potentials to said X-ray tube whereby current is caused to flow through said discharge path, means for producing a voltage proportional to the current through said discharge path, a condenser, a variable resistor, connections for charging said condenser through said variable resistor from said voltage proportional to the current through said discharge path, a gas-filled discharge tube connected to said condenser so as to be ignited when the voltage across said condenser reaches a predetermined value, and means operable in response to the ignition of said discharge tube for opening said connections for supplying operating potentials to said X-ray tube.

2. Apparatus as claimed in claim 1, in which said connections for supplying operating potentials to the X-ray tube include a manually oper- 

